1. Field of the Invention
This invention has widespread usage in communication systems wherein the transmission links exhibit variable amplitude and delay characteristics. Typical usage includes commercial, military, and foreign data communication systems over telephone lines.
2. Description of the Prior Art
Data modems capable of transmitting data over ordinary voice grade telephone lines are well known in the art. Such telephone lines exhibit amplitude distortion, phase jitter, delay distortion, phase hits and frequency translation. Because of these adverse conditions caused by the telephone lines, modems require proper equalization for accurate data transmission. As a result of the noise problem, modem equalization, i.e. compensation to correct for and balance out noise caused by the telephone line has become a sophisticated technology.
Initially, signals received at a modem receiver from a modem transmitter were viewed on a scope by an operator while the operator performed manual equalizing adjustments. The data signals when received at an equalized receiver, presented on the scope a particular shape known in the industry as an eye pattern. The use of the eye pattern was not acceptable because the equalization results were difficult to achieve except by highly skilled operators.
The assignee of this invention developed novel techniques of manual and automatic equalization using a frequency domain equalizer in which the scope and eye pattern technique were replaced by a null meter. The null meter depicted a low reading whenever the modem receiver was well equalized. The simplicity of manual and automatic equalizers including the manual and automatic adjustment approaches are fully described in the U.S. Pat. Nos. 3,550,005; 3,566,271; 3,742,360 and 3,644,830 and references cited therein.
Automatic equalizers operating in either time or frequency domain are well known in the art. Such equalizers receive a data stream and automatically adjust signal taps that influence characteristics in order to compensate for distortion caused by telephone lines and other circuit parameters. Generally in the prior art such equalizers start from a "scratch", or non-equalized condition, each time a new data message is received. Such messages are preceded in the prior art by a test sequence which allows the equalizer to adjust the signal taps, in a proper manner for equalization to occur. Data throughput is reduced in the prior art by requiring completely fresh equalization prior to each data message. The myriad of operational conditions that must be taken into account in data modem operation has heretofore prevented fully automatic operation of modems controlling their own equalization and data sequences with reduced equalization time.